This invention relates to a rotary control valve of a power steering system of an automotive vehicle comprising a torsion bar connecting an input shaft and an output shaft together, a spool valve member secured to the input shaft and a valve body loosely fitted to the valve member and secured to the output shaft, the spool valve member and the valve body both being formed with axial grooves for supplying and discharging a pressure fluid from a hydraulic pump and each axial groove being formed with control edges for effecting control of the area of an opening defined by the axial grooves and the flow rate of the pressure fluid by the relative angular displacement of the spool valve member with respect to the valve body as the torsion bar is twisted by a load applied to the output shaft, and more particularly it is concerned with the spool valve member and a method of producing same.
In this type of rotary control valve, the input shaft is connected to a steering wheel of an automotive vehicle and the output shaft is connected to a pinion when the rotary control valve is used with a power steering system of a rack-and-pinion arrangement. When the rotary control valve is incorporated in a power steering system of a ball-nut arrangement, the input shaft is connected to a steering wheel of an automotive vehicle and the output shaft is connected to a ball-nut shaft, as is well known. The spool valve member of the rotary control valve is operative to supply a pressure fluid to a hydraulic cylinder for hydraulically actuating the output shaft of the power steering system or to discharge same therefrom in accordance with the direction in which the automotive vehicle is driven, from a hydraulic pump which is driven for operation by an internal combustion engine of the automotive vehicle, for example. In this case, it is necessary that the volume of the pressure fluid supplied to and discharged from the hydraulic cylinder be increased to reduce the force necessary for manipulating the steering wheel in the range of low engine speed as the automotive vehicle travels through the town or is put into its garage or is brought to a stop and reduced to zero to increase the force necessary for manipulating the steering wheel in the range of high engine speed to ensure the stability of the travel. In view of the foregoing, it is considered necessary that the power steering system have a characteristic such that the automotive vehicle can be driven manually without requiring the pressure fluid being supplied from the hydraulic pump to the hydraulic cylinder. Particularly, it is considered necessary that the rigidity of the steering wheel be increased in the range of high engine speed to obtain improved steering characteristics and the characteristics of the power steering system be improved in the range of minuscule angle steering or the range of low pressure of the pressure fluid. Thus, to meet the aforesaid requirement, various proposals have hitherto been made to provide improvements in the prior art. For example, one of such proposals involves the additional use of spring reaction means for increasing the rigidity of the steering wheel while the automotive vehicle travels straight ahead. Another proposal contemplates the additional use of vehicle speed sensor means to cause the steering force to vary in conformity with the vehicle speed. The proposals are also concerned with the control edges of axial grooves of the spool valve member. One of them envisages forming one or two inclined surface portions of a rectangular shape having a predetermined angle with respect to the outer side of the spool valve member which extend along the entire length of the axial grooves. Another proposal provides control edges simultaneously as hobbing is performed to form the axial grooves of the spool valve member. The last-mentioned two proposals aim at rendering complex in shape the control edges of the spool valve member serving as a control valve member for the pressure fluid. However, some disadvantages are associated with these proposals of the prior art. When the control edges have one or two inclined surface portions of a rectangular shape, it would be impossible to provide a desired characteristic with respect to a rise in pressure from the low pressure range to the high pressure range due to the fact that the area of the opening has a high changing rate with respect to the rotational angle of the spool valve member relative to the valve body. Also, difficulties would be faced with in providing the inclined surface portion or portions by working on the spool valve member. Thus, to obtain the desired control characteristics, it would be necessary to give high precision finishes to parts in various production steps, thereby causing an increases in working costs. Meanwhile, when the control edges are formed simultaneously as the axial grooves of the spool valve member are formed by hobbing, limitations would be placed on the configuration of the control edges not only by the radius of the axial grooves but also by the machining operation which should be performed following the hobbing operation. Thus, the control edges provided by this process could not have the desired pressure control characteristics as would be the case with the control edges having the inclined surface portion or portions described hereinabove. To cope with this situation, a further proposal has been made, as described in Japanese Patent Laid-Open No. Sho-57-198170, for example, to form at least one control edge in each axial groove of the spool valve member to obtain the desired control characteristics. However, this proposal has the disadvantage that, whem emphasis is placed on improving the control characteristics in the range of high engine speed, the control characteristics would deteriorate in the range of low engine speed, and, when emphasis is placed on improving the control characteristics in the range of low speed, the control characteristics would deteriorate in the range of high engine speed. Also, the pressure characteristic would be such that a rise in pressure with respect to a steering torque from the range of low presure to the range of high pressure would show a gentle slope, and it would be impossible to show a linear change over a prolonged period of time while a rise from the range of low pressure to the range of high pressure would be slow. Thus, even if a multiplcity of stages of control edges could be designed that would enable control characteristics substantially the same level as the target control characteristics to be achieved, it would be necessary to give fine finishes to the parts, and difficulties would be experienced in performing operations for forming the control edges which involve changing elevated portions to depressed portions. Also, in performing machining when the spool valve member is worked on to form control edges, it is necessary that the surfaces of the control edges be given with a high degree of precision finishes because the spool valve member is machined in the axial direction when the control edges are formed and the machined surfaces are at a right angle to the direction of flow of the pressure fluid, thereby giving rise to noises as the pressure fluid flows. This would reduce the service life of the cutters and increases the cost of operations.